Headwind, higher burn and the alternate that may no longer be needed
An ATR 72-500 is planned from Nairobi Jomo Kenyatta (elevation 5,330 ft) to Mombasa Moi International (elevation 200 ft), with Malindi as destination alternate. The plan shows taxi 100 kg; trip 1,100 kg for 55 minutes; contingency 5% of trip; alternate 25 minutes; final reserve 30 minutes at holding flow. Cruise flow is 1,200 kg/h and holding flow 1,000 kg/h. Block fuel is 2,400 kg. Thirty minutes after takeoff, fuel remaining is 1,640 kg - 60 kg more used than planned, from a 35 kt stronger headwind than forecast and a 10-minute level-off at the wrong level. The remaining track now requires 30 minutes. Mombasa reports wind calm, visibility 10 km or more, scattered cloud at 3,000 ft, and both runways in service. Malindi is 25 flying minutes away with similar fine weather.
1. Verify the fuel plan as filed. Show the minimum required fuel and the margin.
Read the worked answer
| Item | Calculation | kg |
|---|---|---|
| Taxi | given | 100 |
| Trip | given | 1,100 |
| Contingency | 5% × 1,100 | 55 |
| Alternate | 25 min × 1,200 kg/h (25/60) | 500 |
| Final reserve | 30 min × 1,000 kg/h | 500 |
| Required | 2,255 |
Block fuel 2,400 kg gives a margin of 145 kg - about 7 minutes at cruise flow. That is a slim but legal plan, and it depends entirely on the alternate being required and being available. Note that the alternate is a real cost: 500 kg, over a fifth of the block fuel. The plan is valid only if the destination weather justifies it.
2. At the 30-minute point, do the arithmetic and decide whether to continue to Mombasa or divert.
Read the worked answer
Present position: 1,640 kg remaining.
Requirement if the alternate is still carried:
- Remaining trip: 30 min × 1,200 = 600 kg
- Contingency 5%: 30 kg
- Alternate: 500 kg
- Final reserve: 500 kg
- Total 1,630 kg → margin 10 kg.
That is effectively zero. With the alternate in the plan, the flight is at its absolute limit and any further headwind, vector or hold eats the final reserve.
But the alternate may only be retained when it is required. Mombasa is reported wind calm, 10 km or more, SCT030, two runways in service: no destination alternate is required, so the 500 kg alternate fuel is not part of the requirement. Requirement becomes 600 + 30 + 500 = 1,130 kg, leaving a margin of 510 kg, about 25 minutes at cruise flow. Decision: continue to Mombasa, but with no holding and no discretionary track miles, and tell ATC and the company the fuel state.
3. Define minimum fuel and MAYDAY fuel, and state what you transmit and when.
Read the worked answer
MINIMUM FUEL is the declaration when the fuel on board is such that landing at the destination with the final reserve intact is possible, but any additional delay would result in landing below the final reserve. It is an advisory to ATC, not a distress call; it tells them the aircraft can no longer accept delay.
MAYDAY FUEL is declared when the fuel is calculated to be below the final reserve at landing. It is a distress condition and gives the aircraft priority over all other traffic.
Here: continue, with the fuel state monitored every few minutes against the 1,130 kg requirement. If a hold or extra track is imposed - or the Mombasa weather deteriorates so that the alternate is required again - declare MINIMUM FUEL immediately and ask for priority. If the calculation then shows landing below the final reserve, declare MAYDAY FUEL, and be prepared to divert to Malindi, which from present position needs only about 500 kg plus 500 kg final reserve. Also inform the operator; the excess burn and level-off are company-reportable events.